The use of powder metallurgical techniques has become popular with high alloyed materials due to the problems encountered in casting such materials, e.g., segregation and the resulting loss of physical properties. For example, powder metallurigal techniques are used extensively with nickel, cobalt, and ferrous-base superalloys. These are high temperature-high strength alloys used in making turbine discs, blades, buckets, and other components of turbine engines which are subjected to high stress at mid-range or high temperatures. The very properties which make these alloys attractive for use in turbine engines cause the consolidation of the powder alloys to be difficult. Moreover, subsequent operations, such as forging and machining the resulting densified compact, to produce a final part are also difficult because of the high stength and toughness of these alloys.
Due primarily to the costs and difficulties encountered in post-consolidation processing, efforts have been made to produce "near net shapes." As used herein, a near net shape is a densified powder metal compact having a size and shape which are relatively close to the desired size and shape of the final part. Heretofore, crude performs have been produced which require extensive post-consolidation forming and machining to produce the relatively complex final part. Producing a near net shape reduces the amount of post-consolidation processing required and the amount of scrap generated. For example, in many instances subsequent hot forging can be eliminated and the amount of machining required can be significantly reduced. Since these materials are difficult to machine, a reduction in the amount of machining offers a marked savings in tool and labor costs. Additionally, these materials are quite expensive, therefore, a reduction in machining results in a savings in material costs. Obviously, eliminating or reducing the amount of hot forging also offers savings advantages.
While the desirability of producing near net shapes has been recognized, many problems have been encountered in accomplishing this objective. The basic step of consolidating the metal powder to produce a powder metal compact having a near net shape has been a major obstacle. Once an acceptable near net shape is produced, other problems are presented. One of these relates to the inspection of the near net shape by ultrasonic testing to detect flaws. One of the important uses of superalloy material is for discs and rotors in jet engines. Such components must pass stringent inspection to reduce the possibility of in-service failure. Hence, inspection of such components is an important consideration in their manufacture.
Ultrasonic testing is a well-known nondestructive testing technique for detecting the presence of flaws, particularly inclusions. In ultrasonic testing a beam of acoustical radiation having a frequency higher than the frequency of audible sound is transmitted through the object to be tested. In the simplest forms of ultrasonic testing, a continuous stream of waves is sent through the part under test. Large flaws in the part cast an acoustical shadow on the opposite face of the piece which can be detected. Ultrasonic waves can be produced and detected by piezoelectric crystals.
Smaller flaws are detected by more sophisticated ultrasonic equipment which sends a wave train into the part. Small flaws relect the waves which are detected and displayed on a cathode-ray tube or recorded by a suitable recording device.
In any ultrasonic testing procedure it is necessary to provide a smooth surface on the part. A surface having the smoothness required for a particular ultrasonic apparatus is commonly referred to as a "sonic surface." In order to produce a sonic surface the part is normally ground. While recent efforts have been made to develop ultrasonic testing techniques for parts having curved surfaces, undercuts, and the like, such techniques are time consuming and generally unsuitable for production-scale manufacturing. The easiest part to inspect by ultrasonic testing is one of a simple shape which has flat, parallel surfaces. Inherently, near net shapes produced by previously known methods have neither. Consequently, it has proven difficult to ultrasonically test such near net shapes.